1. Field of the Invention
The present invention relates, in general, to a truss-reinforced spacer grid and a method of manufacturing the same and, more particularly, to a truss-reinforced spacer grid having improved mechanical structural strength and thermal hydraulic performance and a method of manufacturing the same.
2. Description of the Related Art
Nuclear fuel assemblies are charged into the core of a pressurized water reactor. These nuclear fuel assemblies are composed of a plurality of fuel rods, into each of which a cylindrical uranium sintered compact (or a cylindrical uranium pellet) is inserted.
The fuel rods can be divided into two types, cylindrical and annular, depending on the shape. The fuel rods are structurally vulnerable, because the length is very long in relation to the outer diameter. In order to make up for this drawback, a plurality of supports are used.
FIG. 20 is a schematic front view illustrating a conventional nuclear fuel assembly having fuel rods. FIG. 21 is a schematic top plan view illustrating a conventional spacer grid. FIG. 22 is a perspective view illustrating a conventional spacer grid.
As illustrated in FIG. 20, the nuclear fuel assembly 100 includes fuel rods 110, guide tubes 120, spacer grids 150, an upper end fitting 160, and a lower end fitting 170.
Each fuel rod 110 is enclosed by a zirconium alloy cladding tube and has a structure in which the nuclear fission of a uranium sintered compact or a uranium pellet (not shown) generates high-temperature heat.
Each fuel rod 110 has upper and lower end plugs 130 and 140 coupled to lower and upper portions thereof so as to prevent inert gas which has been used to fill up the cladding tube thereof from leaking out.
The structure of the fuel rod 110 is considerably long compared to the diameter thereof. When a coolant flows through this structure having such a great elongation ratio, the fuel rod 110 responds by causing flow-induced vibrations.
Thus, in order to reduce these flow-induced vibrations, spacer grids 150 are installed in a predetermined section selected with respect to the entire length of the fuel rods 110, so that it is possible to reduce the vibrations of the fuel rods 110 caused by the flow of the coolant.
Meanwhile, in the spacer grid 150 as shown in FIGS. 21 and 22, contact portions between an inner grid plate and an outer grid plate, between the inner grid plates, or between the outer grid plates must be welded, and such welding forms beads which increase the magnitude of the pressure drop in the core region, and thus thermal hydraulic performance may become degraded.
Further, coolant mixing vanes are formed on the inner or outer grid plate for the purpose of mixing the coolant in the sub-channels where the coolant flows or between neighboring nuclear fuel assemblies. These coolant mixing vanes are another cause of the magnitude of the pressure drop in the core increasing. For this reason, although these coolant mixing vanes are supposed to improve the thermal hydraulic performance of the coolant, they may reduce cooling performance instead.
In addition, the spacer grid 150 undergoes a local buckling phenomenon at the outer shell (structurally, the weakest point) when a side receives an impact, so that it is possible to reduce the buckling strength of the entire spacer grid 150.
Furthermore, the thickness of the grid plate which forms the spacer grid 150, in the state where the pitch of the fuel rod 110 is fixed for the sake of performance of the nuclear function, is extremely restricted by this external condition.
In detail, a grid plate that is too thin may reduce the strength of the spacer grid 150. In contrast, a grid plate that is too thick may reduce the interval between the fuel rods 110 and increase the possibility of a departure from nucleate boiling due to contact being made between the fuel rods or may greatly reduce the performance of the fuel rod 110 due to excessively increasing the magnitude of the pressure drop.
Thus, the spacer grid 150 formed of the aforementioned grid plate makes it difficult to continue to solidly support the fuel rods 110 and to further enhance the thermal hydraulic performance.